Space in industrial and domestic landfills is becoming increasingly scarce. In recent years, there has been a growing awareness that there is a need to conserve space in the existing landfills by reducing the volume of solid waste materials through a variety of means (e.g., recycling, reducing consumption, incineration, or other means).
Waste disposable cellulosic materials (e.g., newsprints, disposable diapers, shopping bags, fast food containers) contribute significantly to the total volume of waste in domestic landfills in the United States. Cellulosic materials are generally defined as those materials that contain cellulose, a polymer of .beta.-D-glucose units. Environmental concerns have prompted major international food suppliers to replace styrofoam packaging materials with cellulosic packaging materials. This has resulted in a large new source of cellulosic waste deposited in the landfills.
There is intense interest in the biomass conversion of waste cellulosic products into alternative fuels. The availability of a commercially feasible means of converting cellulosic materials into a usable energy source would reduce the volume of solid waste deposited in landfills and reduce our dependence on foreign fossil fuels.
To date, efforts directed toward developing means for degrading cellulosic materials using microorganisms have focused on cellulose degrading fungi (e.g., Trichoderma reesei) . However, there are disadvantages to working with fungi. Relative to bacteria, fungi are generally difficult to grow in a fermentor. Fungi are relatively difficult to manipulate by means of genetic engineering; therefore, the potential for enhancement of the cellulose degrading properties of a fungus by means of recombinant DNA technology is low. The use of cellulase-producing fungi in fermentations requires the addition of enzymes to prevent product inhibition of cellulase production, which increases the cost of using the technology.
As a consequence of the disadvantages of working with fungi, scientists in the field began to focus attention on employing bacterial strains in the degradation of cellulosic materials. Most of the efforts have concentrated on anaerobic cellulose-degrading bacteria. However, conducting fermentation reactions in the absence of oxygen is a difficult and expensive procedure, relative to aerobic fermentation.
In general, the bacterial and fungal species that have been employed in the degradation of cellulosic materials do not efficiently excrete cellulase into the surrounding growth medium or the cellulase produced cannot efficiently degrade solid cellulosic materials. As a consequence, chemical modification of cellulosic waste is often required to make the solid cellulosic materials available for biodegradation. This additional step increases the cost of the method, and reduces the environmental advantages to be gained through biodegradation of cellulosic materials.
Another problem that has been encountered in the art is that most cellulase-producing organisms produce cellulase under acidic conditions. Under acidic conditions, the cellulase tends to adhere to filtration membranes, making it difficult to recover the cellulase.
The situations in which cellulose-degrading microorganisms may be used to advantage are commonly associated with conditions that are unfavorable to the growth of most microorganisms. For example, cellulosic waste streams, paper production processes, and fossil fuel often produce extremely harsh environmental conditions. Bioreactor solid matrices are formed under severe conditions, for example, at a very alkaline pH.
Cellulosic waste streams and fossil fuels often contain hazardous materials in addition to the cellulosic products. The cost of bioremediation of waste streams and fossil fuels increases when multiple organisms must be added to the site in order to accomplish disparate goals.
What is needed in the art is a cellulase-producing bacterial strain that is able to tolerate harsh environmental conditions, including alkaline conditions, and which has been genetically engineered through recombinant: DNA technology to produce enhanced levels of cellulase, relative to the unmodified strain.